Thermal inkjet print cartridges operate by rapidly heating a small volume of ink, causing the ink to vaporize and be ejected through an orifice to strike a recording medium, such as a sheet of paper. When a number of orifices are arranged in a pattern, the properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead has moved across the paper. The thermal inkjet printer is faster and quiet, as only the ink strikes the paper. These printers produce high quality printing and can be made both compact and portable.
In one design, the printhead includes: 1) an ink reservoir and ink channels to supply the ink to the point of vaporization proximate to an orifice; 2) an orifice plate in which the individual orifices are formed in the required pattern; and 3) a series of thin film heaters, one below each orifice, formed on a substrate which forms one wall of the ink channels. Each heater includes a thin film resistor and appropriate current leads. To print a single dot of ink, an electrical current from an external power supply is passed through a selected heater. The heater is ohmically heated, in turn superheating a thin layer of the adjacent ink, resulting in explosive vaporization and, consequently, causing a droplet of ink to be ejected through an associated orifice onto the paper.
One prior print cartridge is disclosed in U.S. Pat. No. 4,500,895 to Buck et al., entitled "Disposable Inkjet Head," issued Feb. 19, 1985 and assigned to the present assignee.
In these printers, print quality depends upon the physical characteristics of the orifices in a printhead incorporated on a print cartridge. For example, the geometry of the orifices in a printhead affects the size, trajectory, and speed of ink drop ejection. In addition, the geometry of the orifices in a printhead can affect the flow of ink supplied to vaporization chambers and, in some instances, can affect the manner in which ink is ejected from adjacent orifices. Orifice plates for inkjet printheads often are formed of nickel and are fabricated by lithographic electroforming processes. One example of a suitable lithographic electroforming process is described in U.S. Pat. No. 4,773,971, entitled "Thin Film Mandrel" and issued to Lam et al. on Sep. 27, 1988. In such processes, the orifices in an orifice plate are formed by overplating nickel around dielectric discs.
Such electroforming processes for forming orifice plates for inkjet printheads have several shortcomings. One shortcoming is that the processes require delicate balancing of parameters such as stress and plating thicknesses, disc diameters, and overplating ratios. Another shortcoming is that such electroforming processes inherently limit design choices for nozzle shapes and sizes.
When using electroformed orifice plates and other components in printheads for inkjet printers, corrosion by the ink can be a problem. Generally speaking, corrosion resistance of such orifice plates depends upon two parameters: ink chemistry and the formation of a hydrated oxide layer on the electroplated nickel surface of an orifice plate. Without a hydrated oxide layer, nickel may corrode in the presence of inks, particularly water-based inks such as are commonly used in inkjet printers. Although corrosion of orifice plates can be minimized by coating the plates with gold, such plating is costly.
Yet another shortcoming of electroformed orifice plates for inkjet printheads is that the completed printheads have a tendency to delaminate during use. Usually, delamination begins with the formation of small gaps between an orifice plate and its substrate, often caused by differences in thermal expansion coefficients of an orifice plate and its substrate. Delamination can be exacerbated by ink interaction with printhead materials. For instance, the materials in an inkjet printhead may swell after prolonged exposure to water-based inks, thereby changing the shape of the printhead internal structure.
Even partial delamination of an orifice plate can result in distorted printing. For example, partial delamination of an orifice plate usually causes decreased or highly irregular ink drop ejection velocities. Also, partial delamination can create accumulation sites for air bubbles that interfere with ink drop ejection.